Thousands of residents in Michigan and the surrounding Great Lakes region have been warned to stay indoors as a plume of dangerous toxins blankets the US. 

The US Environmental Protection Agency (EPA) and National Weather Service (NWS) have issued alerts about the air quality over Detroit reaching 'unhealthy' levels Monday morning. 

Elevated levels of PM2.5, microscopic particles composed of toxic compounds or heavy metals from car exhaust and factory emissions, have become trapped near the ground by dense fog and high atmospheric pressure over the city.

This type of particulate matter is small enough to enter and damage the lungs, worsen respiratory issues such as asthma, and even contribute to heart attacks and strokes that cause premature death if you breathe in large amounts. 

Air quality-tracking website IQAir received reports across Detroit, a city of more than 640,000 people, displaying an Air Quality Index (AQI) surpassing 150, and even nearing 170 in some neighborhoods. 

Air quality levels are measured on a scale from 0 to 500: good (0–50) carries little risk, moderate (51–100) may affect sensitive individuals, unhealthy for sensitive groups (101–150) poses increased risk and unhealthy (151–200) impacts everyone, limiting outdoor activity.

NWS officials said in their health advisory: 'It is recommended that, when possible, you avoid strenuous outdoor activities, especially those with heart disease and respiratory diseases like asthma.'

'Monitor for symptoms such as wheezing, coughing, chest tightness, dizziness, or burning in the nose, throat, and eyes. Reduce or eliminate activities that contribute to air pollution, such as: outdoor burning, use of residential wood burning devices.'

NWS added that Americans in the unhealthiest parts of the city should keep their windows closed to prevent smoke and polluted air from entering their home.

If possible, the advisory recommended running an air purifier with MERV-13 or higher-rated filters. The Air Quality Advisory is expected to last through noon ET on Monday.

However, the unhealthy air warnings have spread far beyond Detroit, with Air Quality Index (AQI) readings across Southern Michigan soaring into the 150s and 160s Monday morning.

The poor air quality has been driven by exceptionally dense 'radiation fog' that's blanketed the Midwest and the South, covering parts of Texas, Louisiana, Arkansas, Missouri, Tennessee, Kentucky, Indiana, Illinois, Ohio, West Virginia, Pennsylvania, and Michigan.

NWS has warned that in many of these areas, drivers will have extremely low visibility of less than a quarter-mile on some roads.

While this kind of fog is unrelated to nuclear radiation and generally should not endanger human health, officials have previously warned that dense, low-level fog can also impact air quality by trapping local pollutants close to the surface. 

Radiation fog is quite common throughout the US, especially in areas like California and in the Southeast, as it forms when the ground cools quickly overnight under clear skies and calm winds, turning moisture in the humid air into thick fog at ground level.

Radiation fog is most often seen during the fall and winter months, when longer nights allow for more cooling, cooler temperatures, which help the air saturate faster, and loads of moisture from seasonal storms. 

Despite the widespread fog bank cutting across the US on Monday morning, the worst effects of the ground-level pollution have been focused around Michigan, Ohio, and Indiana.

NWS air quality alerts have been issued near multiple cities, including Cleveland and Toledo in Ohio, and South Bend and Fort Wayne in Indiana.

In Michigan, the EPA has added major cities such as Lansing, Grand Rapids, Kalamazoo, and Flint to the areas with air quality that's 'unhealthy for sensitive groups.'

That means most healthy adults and people without respiratory issues will usually be fine and won't notice much, but children, teens, the elderly, pregnant women, and those with health and lung conditions could get sick with prolonged outdoor exposure.

Air pollutants often build up around highly industrialized cities on days when stagnant air is particularly elevated.

Stagnant air means the air around you is barely moving, like it's stuck under a lid, so pollution, fire smoke, car fumes, and other particles can't blow away or mix higher up in the atmosphere, causing them to build up near the ground where we breathe.

It's aided by high atmospheric pressure, which tends to be higher in the winter and in the morning hours. 

Pressure is measured in millibars, with normal sea-level pressure being 1,013 millibars. Anything between 1,015 and 1,020 millibars is considered a high-pressure system.

On Monday, the pressure over Detroit measured around 1,018 millibars, according to IQAir.